Structural assemblies are commonly formed by joining two or more structural members using fasteners, such as rivets. In the aerospace industry, where weight and strength are of critical concern, the joints of structural assemblies typically are subjected to repeated cycles of shear, compressive, and tensile stresses over the life of the assembly. As a result, the rivets must have good mechanical strength and fatigue resistance without adversely affecting the overall weight of the structural assemblies. In addition, because the structural assemblies may be exposed to the ambient environment, including moisture exposure and temperature fluctuations, the joints must be secured with rivets having good corrosion resistance and resistance to thermal stresses. To address the strength and weight requirements, conventional rivets are typically formed of materials having high strength-to-weight ratios, such as aluminum and aluminum alloys that have been hardened by cold working or precipitation hardening. Advantageously, a number of high strength aluminum alloys are available that are lightweight, and also have relatively high fatigue and corrosion resistance. Unfortunately, when in the hardened condition, high strength aluminum alloys tend to lack the formability that is necessary during manufacture and installation of the rivets, which can result in failure by necking, cracking or tearing.
In seeking to solve the problems associated with poor formability, modifications to the manufacturing process for forming the rivets have been proposed. One such modification includes forming the rivets from an aluminum alloy that is in a soft condition and, thereafter, heat treating the rivet, such as by precipitation hardening, to thereby harden the rivet prior to installation and use. The increase in formability of aluminum alloys in a soft condition reduces the likelihood that the rivet will fail as a result of necking, cracking, or tearing during manufacture. However, heat treating reduces the formability of the rivets which, as noted above, can result in failure during installation. Heat treating also adds an additional step during manufacture, which increases the manufacturing costs of the rivets and resulting structural assemblies.
Accordingly, there exists a need for an improved method for manufacturing rivets. The method should provide rivets having high formability to reduce the likelihood of necking, cracking, or tearing during the manufacture and installation of the rivets. The method also should be cost effective so as not to adversely affect the manufacturing cost of the rivets and the resulting structural assemblies. In addition, the rivets should be capable of being formed from materials that have high strength-to-weight ratios, and that exhibit high fatigue and corrosion resistance, as well as resistance to thermal stresses.